The present invention relates generally to data transmission protocols for transmission of data over a shared downlink channel, and more particularly, to a method and apparatus for increasing the reliability of hybrid automatic repeat request protocols.
The Physical Downlink Shared Channel (PDSCH) in the LTE standard is a time and frequency multiplexed channel shared by a plurality of user terminals. The user terminals periodically send channel quality indication (CQI) reports to the base station. The CQI reports indicate the instantaneous channel conditions as seen by the receivers in the user terminals. During each 1 ms subframe interval, commonly referred to as a Transmission Time Interval (TTI), a scheduler at the base station schedules one or more user terminals to receive data on the PDSCH and determines the transmission format for the downlink transmissions. The identity of the user terminals scheduled to receive data in a given time interval, and the transmission format, is transmitted to the user terminals on the Physical Downlink Control Channel (PDCCH).
Hybrid Automatic Repeat Request (HARQ) is used to mitigate errors that occur during transmission of data on the PDSCH. When the base station indicates that a user terminal is scheduled to receive a transmission on the PDSCH, the user terminal is required to decode the PDSCH and to transmit either a positive or negative acknowledgement (ACK/NACK) to the base station. The ACK/NACK informs the base station whether the data packet was correctly received by the user terminal. If the data packet is correctly received by the user terminal, the base station can proceed with the transmission of new data packets. In the event that the data packet is not correctly received by the user terminal, the base station may either repeat the original transmission or send additional redundancy data to enable decoding of the previously transmitted data packet.
The user terminal may send the ACK/NACK to the base station using one of two possible approaches that depend on whether the user terminal is scheduled to transmit on the Physical Uplink Shared Channel (PUSCH). If the user terminal is not scheduled to transmit on the PUSCH when the ACK/NACK is being sent, the user terminal transmits the ACK/NACK on the Physical Uplink Control Channel (PUCCH). If, however, the user terminal is scheduled to transmit data on the PUSCH, the user terminal uses a portion of the allocated resources to transmit the ACK/NACK on the PUSCH.
The user terminal sends ACK/NACK feedback to the base station only when it has been scheduled to receive data on the downlink shared channel. Due to the nature of the wireless communication channel, it is possible that the user terminal may fail to decode a scheduling message transmitted on the PDCCH. If the user terminal fails to decode the scheduling message transmitted on the PDCCH, the base station will receive user data on the PUSCH where it expects to receive ACK/NACK feedback. There is some possibility in this situation for the base station to misinterpret the user data transmitted on the PUSCH as a positive acknowledgement (ACK) when no acknowledgement was sent by the user terminal. This scenario is referred to herein as the false ACK scenario. In the case of a “false ACK,” the base station will think that the user terminal has successfully received the transmitted packet and will transmit new data the next time the user terminal is scheduled on the downlink. Thus, the user terminal will have to rely on higher layer retransmission protocols (e.g., at the RRC level) to request the missing data, which may result in large delays.
Missed ACKs, though less problematic, may also occur. A missed ACK occurs when the user terminal transmits an ACK which the base station fails to detect. In the case of a missed ACK, the base station will unnecessarily waste system resources retransmitting data to the user terminal which the user terminal has already received.
Prior art attempts to solve the problem of “false ACKs” have focused on increasing the reliability of the ACK/NACK feedback to prevent the base station from misinterpreting user data transmitted on the PUSCH as an ACK. One approach is to increase the number of times the ACK/NACK bit is repeated. In general, increasing the number of repetitions reduces the likelihood of a false ACK where no ACK/NACK feedback was transmitted. However, the number of repetitions required to reduce the false ACK to an acceptable level would significantly reduce the PUSCH resources available for user data and thus decrease throughput. This solution also does not solve the problem of missed ACKs.
Another approach for reducing the number of false ACKs is to designate a reserved bit in the uplink scheduling grant to inform the user terminal whether to reserve resources in the PUSCH for ACK/NACK feedback. More specifically, the base station can set the reserved bit to “1” when it schedules the user terminal and expects ACK/NACK feedback on the PUSCH to instruct the user terminal to reserve resources on the PUSCH for ACK/NACK feedback. If the user terminal fails to decode the PDCCH, it transmits a NACK on the reserved resources. The reservation approach, however, is not applicable in all circumstances, since it relies on the presence of an UL scheduling grant associated with the PUSCH. It is therefore not applicable when the user terminal is performing a non-adaptive retransmission or when the user terminal is transmitting in semi-persistent PUSCH, both of which are expected to be common in LTE. This solution also does not solve the problem of missed ACKs.
Another approach to the problem of false ACKs is to mask the PUSCH CRC bits transmitted by the user terminal with the user terminal identity number if the user terminal is transmitting ACK/NACK feedback on the PUSCH. This approach may interfere with normal HARQ processes and therefore be difficult to implement. First, when the user terminal is retransmitting a previous data block, the PUSCH CRC bits cannot be modified. The PUSCH Block Error Rate (BLER), which is usually in the range of 10-40%, is much higher than that of the ACK/NACK error rates. Hence, even if the user terminal does mask the CRC with user terminal identity number, it is very likely the base station will find that both the masked and unmasked CRC bits fail because the entire PUSCH block in error. Hence, the base station is still constantly facing the same uncertainty problem as in the baseline solution. Secondly, the ACK/NACK feedback signal of a previous PDSCH cannot be unknown until the PUSCH is decoded. This would introduce additional delay in the HARQ run-trip time. As a result, either the number of HARQ processes has to be increased or the entire base station hardware needs to be re-dimensioned.
Accordingly, there remains a need for a new approach to reduce the probability of a false ACK being detected when no ACK/NACK is transmitted by the user terminal.